Mike-wsm
The 80Hz modulation of the carrier forces the first agc loop to turn the gain down, then the lack of signal to the second block forces the second agc loop to turn the gain up and amplify white noise.
Not sure we have shown that 80Hz is actualy pressent, that number came from the rpms and prop blade count.
That said if it is related to vibration/rpm it will likely be to fast for AGC to follow but to low to be audible.
hanflyer did add this in a later post:
The noise can only be described as a white noise, no distinct pitch or tone, no relation to engine RPM except that it appears and disappears within 50rpm of 2200rpm every time. There is a chop to it, but it does not change with RPMs.
The chop could be the AGC circuit(s) chasing their tails, not the actual
modulation frequency which if it is anything under 200Hz or so is likely filtered out. (Does the term "motorboating" ring any bells?)